1. Field of the Invention
The present invention relates to a Raman amplifier for compensating for, by Raman amplification, a transmission loss generated in an optical communication system for executing communication using signal light when signal light is transmitted through the optical transmission path.
2. Related Background Art
In an optical communication system for executing communication using signal light, signal light output from the transmitter and transmitted through the optical transmission path suffers a transmission loss. Hence, the signal light that has reached the receiver has a small power. If the power of the signal light that has reached the receiver has a predetermined value or less, normal optical communication may be impossible because of a reception error. To prevent this problem, an optical amplifier is inserted between the transmitter and the receiver to optically amplify signal light, thereby compensating for the transmission loss generated when the signal light is transmitted through the optical transmission path.
Examples of such an optical amplifier are a rare-earth-element-doped optical fiber amplifier (e.g., Er-doped optical fiber amplifier) that uses an amplification optical fiber doped with a rare earth element and a Raman amplifier that uses a Raman amplification phenomenon in a Raman amplification optical fiber. Unlike a rare-earth-element-doped optical fiber amplifier, a Raman amplifier can obtain a desired wavelength band having a gain by appropriately setting the wavelength of pumping light for Raman amplification.
For a WDM (Wavelength Division Multiplexing) optical communication system for executing optical communication by multiplexing signal light components with multiple wavelengths in a predetermined signal light wavelength band, it is important that the gain spectrum of an optical amplifier in that signal light wavelength band is flat. Otherwise, even when a signal light component having a certain wavelength in the signal light wavelength band can be normally received by the receiver, another signal light component having a different wavelength with a small gain may cause a reception error. A technique for flattening the gain spectrum of a Raman amplifier has been studied.
For example, in a Raman amplifier gain flattening technique described in reference 1 “Y. Emori, et al., “100 nm bandwidth flat gain Raman amplifiers pumped and gain-equalized by 12-wavelength-channel WDM high power laser diodes”, OFC′99, PD19 (1999)”, light components output from N (N≧2) pumping light sources are multiplexed and supplied to a Raman amplification optical fiber as Raman amplification pumping light. The gain spectrum of the Raman amplifier is flattened by appropriately setting the output central wavelengths and output powers of the N pumping light sources. In reference 1, the number N of pumping light sources is 12.
In a Raman amplifier gain flattening technique described in reference 2 “F. Koch, et al., “Broadband gain flattened Raman amplifier to extend operation in the third telecommunication window”, OFC′2000, ThD, FF3 (2000)”, the gain spectrum of the Raman amplifier is flattened using a gain equalizer which has a loss spectrum with almost the same shape as that of the gain spectrum of the Raman amplification optical fiber.